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Total Carbonates in Soil

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Abstract

The total carbon content of soils can include inorganic carbonates as well as organic carbon, and inorganic carbonates can interfere in the measurement of organic carbon using elemental analyzers. Soil samples can be acid-treated to eliminate carbonate interference before measuring organic carbon, but this has to be done carefully to avoid potential losses of organic nitrogen, which is typically measured on the same sample. This protocol describes a semi-quantitative assay for carbonates that serves to screen for their presence, indicating the necessity of sample acid pre-treatment, and estimates the amount and likely mineral form of the carbonates. It is based on the “pressure calcimeter method” of Evangelou et at. (1984) and Loeppert and Suarez (1996).


Carbonate minerals, including the common forms calcite and dolomite, release CO2 in reaction with a strong acid:
Calcite: CaCO3(s) + 2HCl → Ca2+ + 2Cl- + H2O + CO2(g)
Dolomite: CaMg(CO3)2(s) + 4HCl → Ca2+ + Mg2+ +2Cl- + 2H2O + 2CO2(g)


To measure the total carbonates, a strong acid is added to a subsample of a ground dried soil in a closed vial, and the change in pressure in the vial is measured in two intervals. A significant pressure increase is assumed to be due to carbon dioxide from carbonate dissolution, with calcite reacting faster than dolomite. Calibration is based on standards for both calcite and dolomite.


The protocol has been modified to detect low levels of carbonate. Sample size should be reduced and/or vessel volume increased for soils high in carbonates, and care should be taken to avoid over-pressurizing the vessel, which could result in its failure.


Sampling frequency: depends on experiment and research purpose.

Protocol

Materials

    • MKS Baratron Type 122A Absolute Pressure Gauge [according to Dick Jacobs of MKS Instruments (jacobsd@mksinst.com) only the Inconel (Nickel/Chromium alloy) sensor will be exposed to the acid vapor; minimize exposure with the acid in the sample and keep the system as dry as possible to prevent damage to the transducer.]
    • Needle adapter coupling for Baratron, consisting of a Cajon reducer fitted to a cutoff ¼-inch-dia. Tuberculin syringe tip and 20G 1.5-inch needle. Be sure this adapter is leak-free before each use; to check, attach and pressurize a vessel to about 1000 Torr with the gas syringe, remove gas syringe/needle, and monitor changes in pressure.
    • Stopwatch or clock
    • Data sheet/book
    • Safety glasses, lab coat, rubber gloves
    • Fume hood
    • 12-mL Labco Exetainers with screw caps and septa (or similar vessels)
    • Glass syringe for acid addition; 10-mL volume, with 24G or 22G 1” needle.Be sure to rinse syringe parts thoroughly with water at the end of the day to minimize corrosion of the metal parts.
    • 5 M hydrochloric acid
    • Standards (see below)

Sample preparation

  1. Soil samples are air dried, passed through a 2 mm sieve, then finely ground (< 0.5 mm diameter) using a pulverizer or similar mechanical device to facilitate reaction with acid.
  2. Subsamples of samples prepared for total C and N analysis can be used in this procedure.

Soil standards

  1. Prepare standards using ground soil free of calcite/dolomite; Add ground calcite/dolomite by weight and % purity as follows
    a. Blank (0%/0% calcite/dolomite)
    b. 0.25% calcite (0% dolomite)
    c. 0.50% calcite (0% dolomite)
    d. 1.00% calcite (0% dolomite)
    e. 0.25% dolomite (0% calcite)
    f. 0.50% dolomite (0% calcite)
    g. 1.00% dolomite (0% calcite)
    h. 0.50% calcite/0.50% dolomite
  2. Agricultural liming materials can be used as standards, but the calcite and dolomite content should be verified using flame atomic absorption spectroscopy.

Procedure

  1. Weigh approximately 1.0 g (0.98-1.02) of dry ground soil into a tared vial. Record the exact weight of sample. Seal the vial with the cap and septum.
  2. Prepare 2 analytical replicates per soil sample. Also prepare a couple of vials containing ~1.0 g of carbonate-free soil or use empty vials to serve as blanks. Also prepare 2 analytical replicates per calcite and dolomite standard.
  3. Wearing safety glasses, rubber gloves, and lab coat and working in a fume hood throughout procedure, inject 2 mL of the 5 M HCl into the vial and vortex for 1-2 seconds to mix. Make sure to point the vial cap away from you in case the septum blows out. Record the time.
  4. After ~10 minutes, mix the solution again and measure the pressure using the transducer. Record pressure. This is a measure of the dissolution of calcite in the sample (the 10-min period may be necessary for full dissolution).
  5. Remove the vial from the transducer. Briefly open the vial to release the excess pressure (wearing safety gear and working in a fume hood!), then seal the cap again. Store at a stable temperature.
  6. After 2-4 hours, measure the pressure again using the transducer. Record the time and pressure. This is a measure of the dissolution of dolomite in the sample (the pressure increase between 10 min and 2 hours is most likely due to dolomite dissolution).
  7. For samples whose change in pressure exceeds the maximum range of the pressure transducer, repeat the procedure with 0.5 g soil and 1 mL of acid, and if that still exceeds the limit repeat with 0.2 g of soil and 0.2 mL of acid.

Calculations

Calibration is based on the pressure response for the calcite and dolomite standards vs. the absolute amount of carbonate. Carbonate concentrations can be expressed on either a mineral or carbon basis. Results can be expressed as % of total dry weight using the amount of soil in the sample.

  1. First calculate the “background-corrected” change in pressure (P) at 10 min. The “background-corrected” P accounts for the increase in pressure due to injection of liquid acid into the closed vial.
    a. Subtract P at time 0 from P at 10 min.
    b. Calculate the average “background” change in P in the blanks from time 0 to 10 min.
    c. Subtract the background average from the result in step 1a to yield the “background corrected” P.
  2. Determine calcite in samples from the “background corrected” P from in step 1c and the calibration. Grams calcite can be converted to % calcite by dividing it by the mass (g) of soil in the sample vial.
  3. Dolomite is determined similarly from the P measured after 2-4 hrs, except those P values do not need to be background corrected because the pressure was released after the 10 min. reading.

References:

Evangelou et al. (1984), An automated manometric method for quantitative determination of calcite and dolomite. Soil Science Society of America Journal 48: 1236-1239.

Loeppert, R.H. and D.L. Suarez. (1996) Carbonate and gypsum. Pp. 437-475 in: Sparks, D.L. [ed.], Methods of soil analysis, Pt. 3 – Chemical methods. SSSA Book Series No. 5.

Date modified: Tuesday, Oct 24 2023

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